It is necessary that we improve both the design and features of the home in order to achieve our net zero goal. Trying to compare our net zero home to the typical American home is like comparing apples and oranges, so we need to come up with a basic home design that we can model against itself in order to see how each of our strategies improves its performance. So our first improvements to our typical American home are to create the baseline energy model so that we compare each material, method and system performance to the same basic floor plan (identical footprints) and even some of the same types of features. In fact, the IECC requires that the HVAC systems used for the baseline home and the energy-efficient improved version (in our case, the zero energy home) are the same basic type. In our case, we are using heat pumps. Our location will be the same for both the baseline and our zero energy homes — outside of Austin, Texas.
To develop our baseline home, we are going to look at improving the home design. We start by looking at the right size of home to fit our needs. Keeping the bigger picture in mind, we think about our long-term needs and come up with a home that is designed to morph over time as we expect our needs to change. We want a home that we can initially raise a family in, but that will not be too large after the kids are grown and gone.
Because we want to follow the “Not So Big House” design considerations discussed in House Designs Category, we are going to start by reducing the conditioned space of the home. Since our goal is net zero energy, we want to design a structure that will allow us to maximize the thermal envelope. So we design a basic rectangle, on two-foot modules. This simplifies the design to a 40-foot-wide × 28-foot-deep footprint, which gives us an improved shape factor (foundation perimeter2/foundation area) of 16.5 and reduces our conditioned space to 1,816 square feet. The home is a three-bedroom, two-bath and has a cooking appliance, a dishwasher and refrigerator, as well as an indoor washer and dryer, just like our typical American home. However, unlike that home, it has a detached open-air carport in lieu of the attached two-car enclosed garage. This not only reduces our construction costs, but also improves our indoor air quality by separating the garage source pollutants from our living space.
For modeling purposes, we are using the 2009 IECC codes for both the baseline home construction specifications. The wall framing is typical wood construction, with overuse of studs at intersections and openings by inappropriately (but typically) trained workers. The walls and floors will have fiberglass batt insulation installed to an ENERGY STAR Grade III quality (the unfortunately typical level of gaps, voids and compressions). The ceilings will have R-30 blown fiberglass. The windows have a U-value of 0.65 and solar heat gain coefficient (SHGC) of 0.30. It is an all-electric home.
This baseline home has a 13-SEER and 7.7-HSPF heat pump split system air conditioner/furnace in the ventilated unconditioned attic space. We made the decision to install a heat pump rather than a gas furnace because gas furnaces on the market today are not very efficient, even those with higher AFUE ratings (i.e., 95 percent efficient). In energy modeling, if we convert gas and electricity to the same units of measurement, high-efficiency electric heat pump systems always easily win hands down.
According to ASHRAE ventilation defaults, a three-bedroom home is considered to have four occupants, one in each secondary bedroom and two in the master bedroom. For water usage, we used the IECC 2009 “Specifications for the Standard Reference and Proposed Designs” baseline of 30 gallons a day plus 10 gallons per bedroom, or a total of 60 gallons per day. For water heating, we have installed one 40-gallon electric water heater, rated at 0.92EF. The lighting is incandescent, and it has standard builder-grade electric appliances.
So, essentially, in our baseline home we have:
- Reduced the conditioned space of the home and detached the parking structure
- Simplified and improved its footprint and its basic design
- Upgraded to an air-source heat pump HVAC system, in lieu of the typical American home’s electric-resistance or oil-burning furnace
- Met 2009 IRC and 2009 IECC code minimums for all systems, construction specifications, lighting and appliances
To finalize our baseline home design we need to examine the effect of its windows on our annual consumption. The typical American home, from the perspective of a building designed for a typical production homebuilder, has window placement for architectural balance on an otherwise drab box design. This home has minimal code-required overhangs and no shading devices to accomplish the desired goal of letting in lots of natural light, again to add interest to otherwise drab interior finishes. When we tallied up the window sizes and placement for this scenario, we found the total window glazing to floor area is about 18 percent.
For most energy efficiency programs, the threshold recognized for limited glazing in a design is 15 percent, yet in the real world, we see many homes built with nearly full-glass walls, far exceeding this recommendation. Yet, even without reducing our glazing area below 18 percent, we were able to reduce the home’s annual energy consumption to 16,267 kilowatt hours. What a difference the reduction in home size and shape factor improvements made! Even with our smaller home’s power density penalty, we still reduced our energy use by 16 percent compared to the typical American home. Again, since our goal is net zero energy, we must reduce the glazing area in order to improve the overall thermal performance of our wall assemblies. We want (and need) window placement for daylighting, passive ventilation and views. So now we are approaching glazing from a functional perspective, so that we can actually use windows for the benefits they can provide, not just as architectural elements. As a result, we were able to reduce our total glazing area to around 159 square feet, which is about eight percent of our conditioned floor area. This revised approach to glazing reduced our total glazing area by more than half!
We can add this feature to our baseline specification:
- Reduced total glazing area to 8 percent of conditioned floor area without any shading devices or improvements to windows ratings
The result of reducing glazing alone, even with a worst-case orientation, reduced our annual energy consumption to 15,646 kilowatt hours! Those 621 kilowatt hours of savings may not seem like much, but considering that we have not yet made any improvements to reduce the heat gains in our building envelope, it’s a good start. This is our baseline annual energy consumption defined by design and building specifications that we can use for comparison to measure the construction and system improvements that will achieve our net zero energy goal.
